In this work the efficiency of several instability indices as summer convective event predictors of high accumulated surface precipitation observed in the Metropolitan Area of São Paulo (MASP) is discussed. During January 28 and February 04, 2004, 5 of 8 events were considered rainy, and associated to thunderstorms at early evening. The K index values were lower than 31 on the 5 observed events, which were affected by the presence of a cold and dry layer on the middle atmosphere level relative to lower levels. The Total Totals index (ITT) failed in detecting 3 of 5 rainy events presenting lower values than the threshold value (ITT=44) expected for heavy showers during all the period. The Lifted index, in the range -4.9 to -4.3, indicated moderate instability for the 5 days. The Showalter index pointed to possibility for severe storms in 4 of the 5 events. The Showalter index and CAPETv values were strongly reduced in the presence of an isothermal layer observed between 910 and 840 hPa in one of the radiosonde measures. The temporal series of CAPETv and Lifted index have shown a good phase agreement, with high linear correlation between them. CINETv ≈ 0 J kg-1, in association to low vertical shear, and Showalter index, Lifted index and CAPETv moderate values, seem to be a common characteristic in days with heavy showers in MASP when large-scale forcing is weak. The need to quantify the probability of detection, frequency of misses, false alarms and frequency of errors in null events for each index is discussed.